When purified 14S mRNA for light chain of immunoglobulin is translated in a reticulocyte lysate and in Xenopus oocytes, two major differences are observed: (1) In the lysate 14S RNA competes efficiently with endogenous mRNA whereas in the oocyte it is translated without reducing the synthesis of endogenous proteins. (2) The translation product of 14S light chain mRNA in the lysate is a protein about 20 amino acids longer than light chain whereas in the oocyte it is a chain of the exact size of authentic secreted light chain. This difference can be explained if 14S mRNA codes for a precursor protein, which is not cleaved in the lysate but can be efficiently converted into light chain in the oocytes.